Certain organic thin films emit light under the action of electric field. If such organic thin film whose thickness is of the order of wavelength of light is sandwiched between a cathode and an anode and applied with a voltage in the range of 5–7 V, substantial amount of light can be generated. Organic light emitting diodes (OLED) are fabricated exploiting this phenomenon of light emission, usually called Electro-luminescence. The basic mechanism, by which the light emission takes place in an organic layer, is through the recombination of electrons and holes in the organic layer. The electrons come in to the organic layer from the cathode that emits and injects electrons in to the organic layer with the help of externally applied electric field. Similarly holes come in to the organic layer from the anode which injects holes in to the organic layer under the action of externally applied electric field. On either side of the organic layer, that emits light, there are other organic layers that transport electrons from the cathode and holes from the anode. For efficient operation of OLED, all the electrons and holes that reach the emissive organic layer should recombine to produce light. Holes need to go up to the emissive organic layer and not beyond. Similarly electrons need to go up to the emissive organic layer and not beyond. Electrons and holes going beyond the emissive organic layer are considered to be a loss because they do not take part in recombination and subsequent emission of light.
To prevent holes from going beyond the emissive organic layer, a ‘hole blocking’ organic layer is deposited adjacent to the emissive layer, as per the prior art. To prevent electrons going beyond the emissive layer, the hole transport layer, adjacent to the emissive layer towards the anode side, acts as a barrier to electrons. In a practical device, holes and electrons do leak towards the electrodes due to high fields (>106 V/cm) existing in the device.
This invention relates to the application of electrical bias to the interior of organic stack to repel the holes and electrons crossing the emissive layer and forcing them back to the emissive layer for effective recombination.
Prior arts have dealt with the problem of leakage of holes and electrons beyond the emissive layer relying on the band-gap of hole-transport layer, for electrons, and hole blocking layer, for holes. In one prior art of 2000 (U.S. Pat. No. 6,097,147) Baldo et.al described a ‘hole blocking’ layer employing thin film of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinr (BCP). This BCP layer had the band gap energy>the energy level of ‘excitons’ (excited neutral molecule as a result of electron-hole recombination) formed in the emissive layer. This BCP layer was formed between the electron transport layer and emissive layer. However at high operating currents, needed for high brightness, the holes leak through and get collected by the cathode. Similarly the electrons leak through the barrier created by hole transporting layer on the anode side and get collected by anode. Hence barrier offered by the ‘blocking layer’ is not sufficient at high operating currents. In another invention by Parthasarathy et.al (U.S. Pat. No. 6,639,357) a similar ‘hole blocking’ layer which also functions as electron transport layer and electron emission layer was employed. The layer is the same as BCP but doped with Lithium (Li) for electron emission. This Li doped layer, in addition to having the same problem of leaking holes to the cathode at high operating currents, also has a problem of Li drift to the emissive layer during operation. Unlike the prior inventions, the present invention controls the loss of electrons and holes through an electrical bias.